Some electrical systems use AC electric motors that are from time to time operated at horsepower levels below that for which they are rated. These motors may be connected to an electric power grid through a variable frequency AC drive, which includes an active converter having a predetermined maximum apparent power capacity. These systems may include one or more motors and one or more variable frequency AC drives. Other electric equipment such as transformers may also be included in the system. For example, in oil fields hundreds of motors driving pumps are connected to the power grid. Some are connected directly to the power grid and others are connected to the grid through individual drives. The system has a power factor that is less than unity (1), but it varies with the changing loads over time as power is demanded from the grid upon operation of the electrical equipment. A leading power factor indicates a circuit that produces reactive power. A lagging power factor indicates a circuit that consumes reactive power. In general, operation of an entire electrical power grid in a leading power factor condition is undesirable and can lead to instability. It is desirable to operate with a slightly lagging power factor, which is close to unity, for example, substantially from 0.93 to 0.97. If the power factor is much below 0.93, current is high and energy may be wasted.
Many variable frequency AC drives use an active converter typically employing insulated gate bipolar transistors (IGBT's) to control and conduct electrical current. In many applications the lifetime of an IGBT is determined by the effects of power cycling. Modern, high power IGBT's are complex devices using silicon dies and substrates. Common failure modes are bond wire fatigue or cracks in a solder layer between the dies and the substrate. This bond wire fatigue can be accelerated under power cycling conditions. While there exists no industry accepted standard to describe “power cycling,” in general, power cycling is any method or mode of operation which causes the level of current conducted through the IGBT to change substantially from a high level to a low level in a relatively short time frame, for example, substantially from 3 seconds to 5 minutes.
Power cycling detrimentally affects IGBT lifetime because it results in thermal cycling of the IGBT. Although power cycling results in thermal cycling, thermal cycling can be caused by other factors. For example, thermal cycling can also be the result of inadequate or inconsistent cooling of IGBT's. Thermal cycling can also be the result of ambient temperature conditions that change cyclically. Therefore, thermal cycling and power cycling are related, but not identical. Power cycling, and the resulting thermal cycling, produces shortened lifetimes of IGBT's. There are many manufacturing techniques and materials that are used to reduce the detrimental effects of power cycling on expected lifetime of IGBT's.
The term “power cycling” is somewhat misleading. A high level of current flow through the IGBT is what causes the IGBT to become hotter. A low level of current flow through the IGBT is what causes the IGBT to become cooler. Therefore, it is a pattern of a high level of current flow followed by a low level of current flow that cause the thermal cycling that can lead to shortened lifetimes of IGBT's. Therefore, power cycling would be more accurately described as “current cycling.” However, the term “power cycling” has become the industry accepted term to describe the phenomenon described herein. AC current measured in a power distribution system (power grid) is composed of both real power components, comprised of current that has a phase difference of 0° with respect to voltage, and reactive power components, comprised of current that has a phase difference of 90° with respect to voltage. Power cycling describes current flow and includes both real power and reactive power, which are discussed in greater detail in “APPENDIX A—TUTORIAL” attached.
The specific location where the temperature is relevant in regard to shortened IGBT lifetime is the bond wire junction between the die and the substrate. However, in most circumstances using IGBT's in industrial applications the bond wire junction is not equipped with a sensor that measures temperature. In most IBGT applications what is measured is something called “case temperature.” Most IGBT's are equipped with a sensor embedded or mounted on the case that encloses all of the semiconductor and other electrical components that comprise the IBGT. Therefore, the relationship of case temperature and bond wire junction temperature must be known or approximated.
The diagram of FIG. 1 shows an example of the transient temperature response during thermal cycling that occurs as the result of power cycling. In this diagram Tj is the junction temperature. Tc is the measured case temperature measured under the hottest die. Tcorner is the measured case temperature at the corner of the substrate solder layer. As can be seen in the above diagram Tj changes much more rapidly than Tcorner or Tc during power cycling. During power cycling Tj can be as much as 2 times greater than Tc and 3 times greater than Tcorner. However, during cooling of the IGBT the difference in temperature measured amongst Tj, Tc and Tcorner is very small. In the above graph, the temperature axis is scaled at 10° per division. Therefore, the change from its minimum temperature to maximum temperature of Tj is 72° C. The change from minimum temperature to maximum temperature is called ΔT (pronounced delta tee). In the above graph: ΔTj=72° C.; ΔTc=33° C.; ΔTcorner=22° C.
It is ΔT under power cycling operating conditions that causes shortened lifetime of IGBT's. Much research has been done on the effects of power cycling on the lifetime of IGBT's. It is not the overall absolute value of temperature that is at issue in this explanation of power cycling and associated shortened lifetime of IGBT's. The maximum absolute safe operating temperature need not be exceeded. It is thermal cycling, however, even thermal cycling that does not result in temperatures above the maximum safe temperature for the IGBT that shortens lifetimes of IGBT's during power cycling. Let's consider an example: Say for example an IGBT has a maximum safe operating temperature of 130° C. measured at Tc. If the IGBT is operated in a power cycling circumstance in which Tc varies between 65° C. and 125° C. Then the ΔTc=125° C.−65° C.=60° C. An IGBT operated in this manner may be subject shortened lifetime and premature unexpected failure. The IGBT may have a shortened lifetime even though the maximum safe operating temperature is never exceeded.
The graph of FIG. 2 shows how ΔTc reduces the number of power cycling events that can be accommodated in the lifetime of an IGBT. As can be seen in this graph, for a ΔTc=60° C. the IGBT will be capable of a given number of thermal cycles as a result of power cycling. However, as both axes are on a logarithmic scale, if ΔTc=30° C., reduction of 50%, the number of power cycles increases by 2000% or 20 times. In general, any significant reduction in ΔT results in relatively large increase in the number of power cycles the IGBT is capable of Ultimately, this can be summarized as follows: Any significant decrease in ΔTc of an IGBT under power cycling operating conditions results in a relatively large increase in the useful lifetime of the IGBT.
This background discussion is not intended to be an admission of prior art.